The invention is related to the field of composite imaging, and in particular the invention uses a technique called Nodal-Differential Imaging (NDI), where an image obtained with a bright focal spot is subtracted from that obtained with a focal ring.
The resolution of optical imaging systems is limited by diffraction to about half the wavelength of illumination. Hence, the resolution of a visible optical microscope is limited to about 250 nm. A large variety of biologically interesting phenomena occur much below this length scale. It is also of great technological importance to be able to image nanostructures (100 nm and below) for the semiconductor industry and the emerging nanotechnology applications. Although electron microscopy can access these regimes, they have fundamental disadvantages. Electrons cause extensive damage to living cells. Electron microscopes require vacuum for the best resolution, which is incompatible with life. Furthermore, electrons being charged particles are easily affected by stray electromagnetic fields. This leads to high uncertainty in the position of the focused electron beam, leading to artifacts and placement errors. Finally, photons provide numerous contrast mechanisms such as fluorescence that are not accessible with electrons. In general, it is highly desirable to image with photons. The Achilles heel of photons is resolution.